System for determining 3-dimensional angle of approach or incidence of radio waves



March 5, 1946- J. Y. HOUGHTON ETAL 2,395,871

SYSTEM FOR DETERMINING 5-DIMENSIONAL ANGLE 0F APPROACH OR INCIDENCE 0FRADIO WAVES Filed Feb. 1, 1.944

em /m2 INVENTOR K HOUGH TON Q /0b EL. /LP TRICK 1A@JL ATTO NEY PatentedMar. 5, 1946 SYSTEM FOR DETERMINING 3 -.DIl\iEN SIONAL ANGLE OF APPROACHOR IN- CIDENCE OF RADIO WAVES Joseph Y. Houghton, Chevy Chase, Md, andEdward L. Kilpatrick, United States Navy Application February 1, 194.4,Serial No. 520,692

(Granted under the act of March 3, 1883,, as amended April 30, 1928; 370O. G. 757) 12 Claims.

This invention relates in general to radio direction finding systems andin particular to an improved system employing in a new manner an antennaof the type disclosed in application Serial No. 460,426, filed October3, 1942, by Edward L. Kilpatrick, one of the present inventors.

In the prior system referred to, two identical loop-antennae wereemployed in symmetrical coplanar relation on opposite sides of a commonplanar axis, the said planar axis was oriented in azimuth by extraneousmeans, such as a homing antenna, to align it azimuthally with theapparent direction of incidence of a radio wave, the loops were thenturned about the common planar axis to a position in which theinstantaneous voltage difference between the voltages generated in thetwo loops produced a satisfactory signal level for detection purposes,and the coplanar axis was then tilted in the predetermined azimuthalplane until the signal was reduced to a minimum (i. e. a null), at whichtilt angle, the planar axis wa considered to be aligned with thethree-dimensional angle of incidence of the radio wave.

Due to possible inaccuracy in predetermining the azimuthal plane byextraneous means (effects of aberration, etc.) this arrangement wassubject to possible error, since if the plane of azimuth was notaccurately determined the null would be obtained at some position inwhich the unbalanced voltage due to differences in time phase withrespect to the horizontal component of flux reaching the two antennaewas balanced out by a residual unbalanced voltage due to difference intime phase with respect to the vertical component. Thus both factors ofthe 3-dimensional angle were dependent upon the accuracy of theextraneous means for predetermining the azimuthal plane.

The present improvement in method and arrangement aims to obviate theaforesaid difiiculties, and to provide a system in which the indicationof the 3-dimensional angle is rendered independent of any extraneousdetermination of azimuth, and an indication of the true angle ofapproach of the incident wave front assured.

Thus one specific object of this invention is to make one double looptype of antenna system serve for S-directional determination of theangle of incidence of radio waves.

Another object of this invention is to eliminate use of the homing loopantenna employed in the Kilpatrick system for horizontal determinations,and thereby eliminate the aberration error re sulting from such use.

Other objects and features will become apparent upon carefulconsideration of the following detailed description when taken togetherwith the accompanying drawing, the figures of which are designed for thesole purpose of illustration and not as a def nition of the limits ofthe invention, reference for the latter purpose being had to theappended claims.

Fig. 1 illustrates the arrangement of the prior art.

Fig. 2 illustrates one embodiment of the pres-, ent invention.

Fig. 3 illustrates, in side elevation, a modified form of systemaccording to this invention, with the antenna portion turned about avertical axis for clarity,

As a prelude to discussion of the present improvement reference is madeto Fig. l of the accompanying drawing illustrating the prior Kilpatricksystem. In Fig. l, the plane ABCD is the horizontal plane embracing theradio transmitter R, and the plane EFGH is the azimuthal vertical planepredetermined by extraneous means, and intended to embrace thetransmitter R. The planar axi SM, of the system comprising the twoidentical loop antennae, #1 and #2, located in symmetrical coplanarrelation on opposite sides of the axis SM, is pivoted about a transverseaxis P to tilt in the predetermined plane EFGH as shown. The system wasprovided with means, as the crank on axis SM, for rotating the antennae,with the axis tilted in some direction other than direction PQ until adefinite signal was obtained. The system was then tilted about axis P toa position in which the signal disappeared, in which position the axisSM was considered to be aligned with the direction of incidence of thewave frcnt (i. e. the direction PQ) and the measure of the inclinationof axis SM, in any suitable manner, exemplifled by pointer T traversingvertical angle scale U, was taken as the vertical angle of incidence ofthe wave front.

Now, if the plane EFGH were accurately determined, then at only onealignment of axis SM, would the radial legs of the two loops besimultaneously cut by magnetic flux of the same time phase, and theywould be so cut in all positions of rotation of the crank. But if theplane EFGH were not accurately determined, then different positions ofthe crank, coupled with different angles of inclination of SM in theinaccurate plane EFGH would produce nulls, and a double possibility oferror thus exists.

Referring now to the embodiment of the present invention exemplified inFig. 2, the system therein shown contemplates continuous rotation of thesymmetrically located identical loops about the axis of symmetry S-M byany suitable means, as by making the axis SM itself the armature of amotor, or, in the form shown, by driving the axis SM from a separatemotor I through a belt 2, shaft 3 and universal joint 1 (shown to makeclear that while constant speed of rotation of the loops may be employedit is not essential). It will be understood that such rotation may bephysical, or by electrical substitution of a number of pairs of fixedloops by commutators or the like. .Under the rotary condition ofoperation, while a minimum (null) position of the loops may be passedthough with an inaccurately located plane EFGI-I and an inaccuratelylocated inclination of SM, this will be only a transient null in therotation of the loops about the SM axis, and a pure minima (nullythroughout the complete rotation employs an suitable means fortranslating the residual voltage difference between the voltages inthetwo loops into an intelligible form, this in the arrangement shown,being effected by coupling the leads from the two opposition connectedloops Ill and H, as through slip rings 9 carried by the shaft 4rotatable about its longitudinal axis SM, to a radio receiver 8a of anysuitable form having its output connected to a translating deviceexemplified by the telephone headset 8. Any suitable means may beemployed for indicating the 3-dimensional angle of orientation of theaxis SM when the pure null is obtained throughout continuous rotation ofthe loops thereabout, herein exemplified by scales 5 and 5' and pointers5 and 6, the latter correlated to follow thehorizontal trace in azimuthand the vertical inclination from horizontal, of the axis SM as it isbrought into alignment with the angle of incidence of the radio wave.

In determining the 3-dimensional angle of incidence of the radio wave bythe present invention, with the aid of the new continuously rotatableloop system of Fig. '2, a new method of procedure is employed. The loopsystem is set into rotation about the axis SM and the receiver adjustedto provide a suitable signal. Then with or without the aid of anypredetermination of the approximate azimuthal angle of incidence of thewave, the axis SM is moved in three dimensions, i. e. swung in azimuthand vertically, thus probing in a cone of decreasing average differenceof voltage phase in the two loops until the alignment 'of pure nullreception is attained, at the single determinate angle in which the axisSM is normal to the incident wave front. The azimuthal angle may then bedetermined, by suitable means, as indicator 6 correlated with scale 5,which may be the scale of a compass card, and the vertical anglelikewise, as by indicator 6' correlatedwith the scale 5' which may bethe scale of an artificial horizon device to refer the vertical angle tothe horizontal plane BC. In this manner the 3-di- "mensional angleof theincident wave front, and thus the apparent location of thewave-originating transmitting-antenna R, is easily and quickly obtained,in a completely determinate manner, and. without introduction of errorfrom inaccuracy of other equipment.

The invention contemplates the employment of gimbals or any otheruniversal mounting in lieu of the shaft 3 and universal 1', slightmisalignment of one of the loops with respect to symmetry to render thetwo identical electrically in the event of slight variations betweenthem, the employment of any suitable ofitake means in lieu of the sliprings 9, operation with each of the loops turned 90 about its planaraxis parallel to the system axis SM (or less efliciently with each ofthe two loops turned to a lesser angle than 90 about one of its planaraxes while maintaining mutual symmetry) and various other modificationsin details of arrangement and procedure, as will be apparent to thoseskilled in the art from the present illustrative disclosure.

' One modification, for example, is shown in Fig.

3, in which electrical rotation of physically stationary loops isemployed. As therein shown, the antenna system is made up of pairs ofelectrically identical loops lfi ll and l -M physically stationary withrespect to the axis of symmetry I2 (normal to the plane of the paper)which may be a longitudinal axis in an aircraft fuselage represented bythe shell IS, the aircraft or like body constituting a support for theantenna system adapted to orient it in 3-dimensions. As is the case inFig. 2, the pairs of loops in Fig. 3 have their planes located inelectrical symmetry on opposite sides of the axis I2, and preferablyother than normal thereto. The pairs i@ i l and lii l l are spaced fromone another, depending on their number, by an angle of 180 divided bythe number of pairs. In Fig, 3, there being two pairs, the angle betweenthe pairs is 90.

As before, suitable means is provided, for determining, as theorientation of the antenna system as a whole is changed, when'a positionis reached'in which instantaneous voltages of equal magnitude and timephase are delivered by the opposite ones of all of the electricallyidentical loops, thereby to identify the orientation of, the antennasystem in which its axis of symmetry 12 is normal to the plane tangentto the wave front at) (i. e. the plane of the paper in Fig. 3, in whichthe loops are located) and hence aligned with the 3-dimensional angle ofincidence of the wave. It will be perceived that if the antenna systemas a whole is tilted relative to the plane tangent to the wave front, atleast one of the pairs of loops will receive in'its respective loopsvoltages out of time phase witheach'other. For some purposes six loopsarranged in three pairs may be preferred; for simplicity four'loopsarranged in two pairs are shown in Fig. 3.

The fact that instantaneous voltages of equal magnitude and time phaseare induced in the respective loops of each pair maybe detected innumerous ways, the preferred mode being that shown in Fig. 3, in whichthe two loops ID l l (and i li -'4 l of each pair are connected involtage opposition by leads I l -45 (and l4 l5=) to output terminal E6ll (and Ni -ll), symmetrically located so that when the'desiredcondition is reached the minimum (null) vector sum of the voltages inthe two loops is all that appears at the terminals.

. As the present invention provides a method and means for bringing theplane of the antenna system (normal to axis 12) into alignment with theplane determined by the total magnetic flux vector and the vectorresultant of the vertical and azimuthal components of the magnetic flux(as distinguished from its horizontal component transverse to itsdirection of propagation) the leads from terminals Ili I6 li li whenextended normal to the plane of the antenna system, have no potentialdifferences induced in them, and have only minimum potentials appliedfrom the terminals. Hence shielding of these leads, while desirable, isnot essential when they are so disposed.

With the loops of each pair connected in opposition as shown in Fig. 3.,various commutating means may be employed to determine when the systemis oriented so that the loops of each pair are producing nulls. In thepreferred form shown in Fig. .3, the commutating means is suitablyshielded, as at l8 and comprises a set of field coils 19,20,corresponding in number and relative displacement to the number anddisplacement of theantenna pairs Ili -4 l lo -4 l and a pickup coil 2irotatable in the field of the set of field coils so that the electricalequivalent of physical rotation of a pair of loops from position Ill -Il to Ni -4 1 etc. is obtained. The output of coil 2-], in thisembodiment, is suitably coupled to the receiver as by slip rings andbrushes 22, for example.

From the illustrative embodiment herein described, it will be apparentthat further modifications and adaptations may be made, both in themethod and the apparatus, without departing from the invention.

It is to be noted that where it is specified in the following claimsthat the two loops are electrically identical, and located in electricalsymmetry, some variation from absolute identity and symmetiy iscontemplated, so long as such variation does not effect the resultssuificiently to introduce material error (in the angle-indication. andthat similarly the identity of voltages in value and time-phase, and thepurity of nulls, is subject to some variation .from extraneous signals,and is subject to some tolerance, depending upon the accuracy ofangle-determination desired. It is further contemplated that slightvariations in identity of the loops may be in whole or in partcompensated for by slight variations in the symmetry of their location,without departure from the invention.

Further, by use of follow-up mechanisms, or other suitable automaticcontrols, the departure from a null indication may be caused to give asignal, or to initiate an automatic probing device to automaticallyreturn the device to pure null position, or to train and direct otherapparatus.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. The method of determining the plane tangent to a radiant energy wavefront which consists in rotating about its axis of symmetry an antennasystem comprising a pair of electrically identical loops having theirplanes located in electrical symmetry on opposite sides of said axis andother than normal thereto, and orienting said antenna system in3-dimensions until identical instantaneous voltages of equal time phaseare delivered by the two loops in all positions of rotation thereof,thereby positioning said axis of symmetry normal to the plane tangent tothe wave front.

2. A method according to claim 1, in which the rotation of the antennasystem is effected in a physical sense, and in which its orientation isalso effected physically.

3. A method according to claim 1, in which the rotation of the antennasystem is efiected in an electrical sense, and its orientation isefiected physically.

4. The method of determining the plane tangent to a radiant energy wavefront which consists in rotating about its axis of symmetry an antennasystem comprising a pair .of electrically identical loops connected involtage opposition andhaving their planes located in electrical symmetryon opposite sides of said axis, and orienting said rotating antennasystem in three dimensions until the resultant voltage of the oppositionconnected antenna system is reduced to a minimum, thereby positioningsaid axis of symmetry normal to the plane tangent to the wave front.

5. The method of determining the plane ham sent to a radiant energy wavefront which consists in spinning about its axis of symmetry an antennasystem comprising a pair of electrically identical loops connected involtage opposition and located in electrical symmetry on opposite sidesof said axis,'and orienting said spinning antenna system in threedimensions until the resultant voltage of the opposition connectedspinning antenna system is reduced to a minimum, thereby positioningsaid axis of symmetry normal to the plane tangent to the wave front.

6. A loop system, for determining the 3-dimensional angle of incidenceof energy waves, having an axis of symmetry orientable in threedimensions, and comprising a pair of identical loop antennas mounted insymmetrical relation on opposite sides of said axis, means for rotatingsaid loops about said axis, a receiver coupled to receive voltages .fromsaid two loops in mutually subtracting relation, means for probing withsaid antenna system by varying the 3-dimensional orientation of saidaxis of rotation until a position of orientation is reached in which aminimum signal is obtained in all positions of rotation of said loops,and means for determining the horizontal and vertical angles of the saidaxis, which in said position of orientation aligns with and indicatesthe angle of incidence of the energy wave.

7. An apparatus for determining the plane tangent to a, radiant energywave front, comprising an antenna system having an axis of symmetry andembodying a pair of electrically identical loops having their planeslocated in electrical symmetry on opposite sides of said axis and otherthan normal thereto, means for roantenna system being mounted on asupport arranged to orient it as a whole in three dimensions, and meansfor determining, as said orientation is shifted, when an orientation isreached in which instantaneous voltages of equal magnitude and timephase are delivered by the respective loops of the pair in all positionsof rotation thereof thereby to identify the orientation of said antennasystem in which its axis of symmetry is normal to the plane tangent tothe wave front. An apparatus for determining the plane tangent to aradiant energy wave front, comprising an antenna system having an axisof symmetry and embodying pairs of electrically identical loopsphysically stationary with respect to said axis of symmetry and havingtheir planes located in electrical symmetry on opposite sides of saidaxis and other than normal thereto, said pairs being displaced from oneanother about said axis by an angle of 180 degrees divided by the numberof pairs; said antenna system being mounted on a support arranged toorient said antenna system as a whole in three dimensions; and means fordetermining, as said orientation is shifted, when instantaneous voltagesof equal tating said pair of loops about said axis, said magnitude andtime phase are delivered by the opposite ones of all of saidelectrically identical loops thereby to identify the orientation of saidantenna system in which said axis of symmetry is normal to the planetangent to the wave front.

9. An apparatus for determining the plane tangent to a radiant energyWave front, comprising an antenna system having an axis of symmetry andembodying a pair of electrically identical loops having their planeslocated in electrical symmetry on opposite sides of said axis and otherthan normal thereto, said loops being connected in voltage opposition tooutput terminals symmetrically located vvith respect to the pair, meansfor rotating said pair of loops about said axis, said antenna systembeing mounted on a support arranged to orient it as a whole in threedimensions, and means for determining, as said orientation is shifted,when a position of orientation is reached in which a minimum of signalis deliveredat said output terminals in all rotative positions of saidloops to identify the orientation of said antenna system in which saidaxis of symmetry is normal to the plane tangent to the wave front.

10. A system according to claim 9, in which said last named meanscomprises a receiver electrically coupled to said antenna system at saidterminals in a manner providing for maintenance of the coupling duringcontinuous rotation of said antenna system.

11. An apparatus for determining the plane tangent to a radiant energywave front, comprising an antenna system adapted to be electricallyrotated about its axis of symmetry and embodying pairs of electricallyidentical loops physically stationary with respect to said axis ofsymmetry and having their planes located in electrical symmetry onopposite sides of said axis and other than normal thereto, said pairsbeing displaced from one another about said axis by an angle of 180degrees divided by the number of pairs and having the loops of each pairconnected in voltage opposition to output terminals symmetricallylocated with respect to the pair; shielded commutator means comprising aset of field coils corresponding in number and relative displacement tothe number and displacement of said pairs, and having its coilsconnected, respectively, across the output terminals of the respectivepairs, and a pick-up coil rotatable in thefield of 'said set of fieldcoilsso that the electrical equivalent of a physically rotated pair ofloops is attained; said antenna system being mounted on a supportmovable to orient said antenna systemas a whole in three dimensions; andmeans for determining, as said orientation is shifted,

when a position is reached in which a minimum of signal is received bysaid pick-up coil in all positions thereof thereby to identifytheorientation of said antenna system in which said axis of symmetry isnormal to the plane tangent to the wave front.

12. An apparatus according to claim 7, in which said pairs of loops aremounted in a plane trans-

